JP6.15 Role of Continental-scale Landmass in Monsoonal and Global Precipitation Distribution

Thursday, 11 June 2009
Stowe Room (Stoweflake Resort and Confernce Center)
Winston Chao, NASA/GSFC, Greenbelt, MD

It was argued by Chao and Chen (2001) that land-sea thermal contrast on the continental scale is not a necessary condition for monsoons and that a monsoon is an ITCZ that has moved into the subtropics in its annual cycle of latitudinal movement. Chao and Chen supported their contention by GCM experiments in which they replaced landmass by ocean and were able to simulate monsoons. However, land-sea thermal contrast does exist and must play a role in monsoonal rainfall distribution. Land-sea thermal contrast is but one facet of continental-scale landmass. The other important characteristic of landmass is its topography. In this article the gross roles of landmass in monsoonal rainfall distribution and in middle latitude storm track rainfall are examined through GCM experiments. Comparison of a set of two GCM experiments in which the observed sea surface temperature (SST) is prescribed with and without a six-month delay reveals the role of land-sea thermal contrast. Another set of experiments, which repeats the first set but with topography of all landmass reduced to zero, when compared with the first set, reveals the role of topography. The GCM used is that of NASA/Global Modeling and Assimilation Office.

These experiments confirm that land-sea thermal contrast is not a necessary condition for monsoons and that a monsoon should be viewed as an ITCZ displaced from a location close to the equator to more than 12 degrees away from the equator, instead of a continent-sized giant sea breeze. However, land-sea thermal contrast and elevated topography do have influence on the distribution of monsoonal rainfall. The temperature rise over south Asia as the season moves into summer helps the Asian monsoon to start or to start early. However, this role is not the same as that of the land-sea thermal contrast as in the conventional explanation for the cause of monsoon. Both heated landmass and elevated topography in the subtropics in summer contributes to the displacement of ITCZ into India and Bay of Bengal. Also, in summer the heated landmass, by drawing moisture toward itself, limits the rainfall of the storm tracks in the middle latitude oceans. On the other hand, in winter the landmass does not present a competition for rainfall and thus allows middle latitude storm tracks to expand over the ocean.

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